Orbital Evolution of Eccentric Low-mass Companions Embedded in Gaseous Disks: Testing the Local Approximation

We study the tidal interaction between a low-mass companion (e.g., a protoplanet or a black hole) in orbit about a central mass, and the accretion disk within which it is submerged. We present results for a companion on a coplanar orbit with eccentricity e between 0.1 and 0.6. For these eccentricities, dynamical friction arguments in its local approximation, that is, ignoring differential rotation and the curvature of the orbit, provide simple analytical expressions for the rates of energy and angular momentum exchange between the disk and the companion. We examine the range of validity of the dynamical friction approach by conducting a series of hydrodynamical simulations of a perturber with softening radius R_soft embedded in a two-dimensional disk. We find close agreement between predictions and the values in simulations provided that R_soft is chosen sufficiently small, below a threshold value Rtilde_soft, which depends on the disk parameters and on eccentricity. We give Rtilde_soft for both razor-thin disks and disks with a finite scaleheight. For point-like perturbers, the local approximation is valid if the accretion radius is smaller than Rtilde_soft. This condition imposes an upper value on the mass of the perturber.